1. Field of the Invention
This invention relates to a paving asphalt concrete composition, and more particularly to an asphalt concrete composition suitable for pavement of motorways, sidewalks, runways, taxiways, parking lots and the like using a modified asphalt-based paving binder composed of a hot asphalt mixture or a stabilized hot asphalt mixture used in a surface course, binder course or a base of a road.
1. Description of the Related Art
From old times, vulcanized rubber crumb (e.g. made from waste tires and the like) have been applied to paving materials as an aggregate or a binder.
As the application as a paving aggregate, there are so-called elastic pavements such as promenades and jogging courses for providing walking comfort by utilizing the elasticity of rubber, anti-freezing pavements such as motorways in snowy and cold areas for breaking a frozen ice layer by utilizing the elasticity of rubber to prevent passing vehicles from slipping, and so on.
As the application as a paving binder, there are a modified paving binder having excellent adhesion and tackiness, which is obtained by mixing vulcanized rubber crumb and aromatic oil with molten asphalt and heating at a temperature above 200.degree. C. with stirring, and so on. The modified binder using the vulcanized rubber crumb (hereinafter referred to as a vulcanized rubber binder) was developed in the United States of America during the 1960s from a viewpoint of recycling waste tires and later introduced even in Europe. At the present time, the vulcanized rubber binder is widely used around Europe and America as a modified binder for porous pavement, an adhesive for a stress absorbing layer sandwiched between the existing pavement and new pavement, or the like.
In case of the paving aggregate, there is a problem that the compaction by rollers is insufficient because the vulcanized rubber crumb dispersed in the vulcanized asphalt mixture acts as a cushion and hence the durability of the pavement is poor. The vulcanized rubber crumb used as an aggregate is mixed in an amount of 3% by weight at maximum with 97% by weight of the vulcanized asphalt mixture. The resulting mixture could only offer the lowest quality to be used for general-purpose motorways, which considerably degrades the pavement durability as compared with the ordinary pavement.
When the vulcanized rubber binder is used as a binder for the hot asphalt mixture, there is derived a greatest problem due to the influence of heat upon the vulcanized rubber in the binder production. That is, when the vulcanized rubber crumb, asphalt and oil are heated with stirring, the vulcanized rubber is swelled and gelated to render into a modified binder having a high viscosity, but as overheating or reheating is repeated, it is softened and becomes extremely unstable to develop no function as a paving binder. In other words, the vulcanized rubber binder is difficult to store under a higher temperature. Therefore, the vulcanized rubber binder should be produced by using a special binder production apparatus in the vicinity of a production plant for the hot asphalt mixture in real time, so that there are many restriction conditions in the production of the hot asphalt mixture.
On the other hand, asphalt containing a usual modified binder is transported from asphalt producing or storing facilities to the production plant for the hot asphalt mixture at a high temperature state and temporarily stored in an asphalt tank of the plant. Thereafter, the stored asphalt can be taken from the tank in an adequate amount at need, so that the degree of freedom in the production of the hot asphalt mixture is very large as compared with the use of the above vulcanized rubber binder.
In case of the production of the hot asphalt mixture containing the vulcanized rubber binder as mentioned above, it is necessary to use a large-size movable mixer capable of producing an adequate amount of the vulcanized rubber binder corresponding to the amount of the mixture to be produced, which requires the increase of installation and labor and hence brings about considerable increase of cost. In such a conventional system producing the vulcanized rubber binder, hot stored asphalt, vulcanized rubber crumb and oil are simultaneously charged into the special large-size mixer with stirring to produce the vulcanized rubber binder, which is transported into the asphalt plant in real time to produce the hot asphalt mixture. Since the vulcanized rubber binder is difficult to be stored under heating, it is considered that such a conventional system is unsuitable for the method of small-lot production destined for a great number of delivery sites commonly adopted in Japan.